The present invention relates to a method for bleaching pulp with ozone. More specifically, the present invention relates to a method for bleaching pulp without reducing pulp quality.
For pulp bleaching, primarily chlorine bleaches have been conventionally used. But it is becoming a matter of concern that environmentally hazardous organic chlorine compounds such as chlorinated dioxins are generated via the decomposition of the organic matters in pulp in chlorination stages. Compared with chlorine bleaching agents having such problems, ozone has a higher bleaching activity and does not generate hazardous substances. Accordingly, ozone has drawn attention for application to pulp bleaching.
In conventional pulp bleaching with ozone, the pulp consistency for contact with ozone has generally been a low pulp consistency such as 1 to 3% by weight pulp or a high pulp consistency such as 30 to 40% by weight pulp. A medium consistency pulp such as 8 to 20% by weight pulp has also been bleached with ozone. The process of bleaching this low consistency pulp with ozone is disclosed, for example, in Japanese Patent Publication No. Sho 53-28723.
It is believed that, when ozone is used for pulp bleaching, ozone induces the cleavage of the glycoside bonds in the cellulose chains, the principal pulp component, lowers the viscosity of bleached pulp and decreases the strength of the final products. To suppress cellulose attack during pulp bleaching with ozone, the following processes, for example, have been proposed.
One process for preventing ozone attack of cellulose involves pretreating pulp with a cellulose protector and subsequently bleaching the pulp with ozone, or bleaching pulp with ozone in the presence of a cellulose protector. Such conventional processes have been disclosed, for example, in Japanese Patent Publication No. Sho 52-6364, Japanese Patent Laid-open No. Sho 55-112390, Hei 1221587, and Sho 53-90403 and Japanese Patent Publication No. Sho 57-53916.
For example, Japanese Patent Publication No. Sho 56-43153 discloses a process of preventing ozone attack of cellulose by bleaching pulp of a higher concentration with ozone of a low concentration of 1 to 15 mg/liter.
For example, Japanese Patent Publication No. Sho 52-14329 discloses a process of preventing ozone attack of cellulose by bleaching pulp with ozone at a low temperature of 5.degree. C. or less.
When a low consistency pulp is bleached with ozone, the reaction rate is slow because the reaction with ozone is via liquid phase, but with the advantage of less ozone attack of the cellulose. However, a low consistency pulp contains a vast amount of water, 97 to 99% by weight, which is costly to handle. Because of a higher water content in the pulp, ozone is more readily dissolved and decomposed in water, with the disadvantage of a lower efficiency of ozone consumption.
When a high consistency pulp is bleached with ozone, the reaction is generally considered to be a reaction between a gas phase and a solid phase, so that ozone directly reacts with the pulp at a high velocity. However, ozone attack of cellulose is distinctive, disadvantageously reducing the strength of the products. Also, the dewatering is difficult, resulting in excessive cost.
The prevention of attack of cellulose, involving pretreating pulp with a cellulose protector and subsequently bleaching pulp with ozone or bleaching pulp with ozone in the presence of a cellulose protector, has a drawback in that the protector is costly.
Because the process of bleaching a high consistency pulp with a low concentration of ozone requires a concentration of ozone of about 1 to 15 mg/liter, the process disadvantageously requires a large amount of an ozone containing gas.
The aforementioned process of bleaching pulp with ozone at a low temperature of 5.degree. C. or less has the drawback of costly cooling.